Nine patients with biopsy-proven primary focal and segmental hyalinosis and sclerosis (FSHS) and steroid-resistant nephrotic syndrome were randomly allocated to either a period of 4-6 months of treatment (ciclosporin; (CS); 5-8 mg/kg/24 h and warfarin) or to a control period (warfarin alone) and then crossed over to the alternative for a further 4-6 months. Serum creatinine levels increased at a similar rate during treatment and control periods of observation. Serum albumin levels increased (p less than 0.05) and urinary protein excretion decreased (p less than 0.01) in association with the CS therapy compared to the control period of observation. No patient had complete resolution of the nephrotic syndrome. In primary FSHS, CS treatment is unlikely to produce complete resolution of nephrotic-range proteinuria but does significantly decrease urinary protein excretion.